1. Field of the Invention
The present invention relates to a communication apparatus, and more particularly a communication apparatus connected to plural terminal units through communication lines.
2. Related Background Art
In such field there is already known a system to be connected, as shown in FIG. 5, to plural telephone units and non-acoustic communication units such as facsimile units through plural telephone lines.
In FIG. 5, two telephone lines (main lines) 2a, 2b are connected to a main unit 2, which can connect three push-button telephone units (touch-tone phones) 4, 6, 8 and a facsimile apparatus 10 to the telephone lines in desired manner.
The telephone unit is not independently connectable to the main line, but a circuit for line control such as making and receiving calls, a channel, a power source etc. are included in the main unit 2. Control of making and receiving call for each telephone unit is automatically executed by the main unit 2.
On the other hand, the facsimile apparatus 10 is usually connectable independently to the line, and, when it is connected to the lines together with the above-mentioned communication system comprising the telephone units 4, 6, 8 and the main unit 2, the lines are switched manually by a switch (not illustrated) or the like. Consequently a signal line 10a connecting the facsimile unit 10 with the main unit 2 is composed of a pair of conductors serving as a communication line only, while each signal line 4a, 6a or 8a connecting each telephone unit with the main unit 2 is composed of two pairs of conductors serving as a communication line and a line for power supply and control.
This conventional system can therefore only achieve the functions of connecting the facsimile unit to the main unit of the telephone system and making a telephone call through a facsimile line when the facsimile unit is not in use. The facsimile unit is so constructed as to be independently connectable directly to the communication line and has a line control function the same as that of the main unit 2, so that an economic communication system cannot be obtained. There is practically no functional combination between the telephone system and the facsimile unit, and it is not possible to control the facsimile unit from the telephone units. Therefore, in case of facsimile communication, the operator has to leave his own desk wherein a telephone unit is placed and to go to the location of the facsimile unit.
Also in facsimile communication in a system shown in FIG. 5 by automatic calling of a destination station with a dial signal generating circuit, if a meaningful signal, or an initial identification signal, cannot be received from the destination station, the station is identified as being unavailable for some reason, and the call is repeated after a predetermined period, for example 1 minute. Then, if a meaningful signal still cannot be received even after a predetermined number of calls (for example 3 calls), the procedure is terminated as an error.
However, if the destination station belongs to a similar communication system in which the facsimile unit is not adjusted at an automatic mode but at a manual mode, communication cannot be made even after repeated calls since a meaningful signal cannot be received. Also if the system does not have a facsimile unit but has only telephone units, an error is identified after a predetermined number of calls are made.
These drawbacks will be encountered also when a data communication apparatus other than a facsimile unit is connected to the line in combination with telephone units.
In a system disclosed in the U.S. patent application Ser. No. 823,286, filed Jan. 28, 1986, of the present applicant, two telephone units and a facsimile apparatus are connected to two telephone lines, to which the telephone units correspond respectively.